1. Field of the Invention
The present invention generally relates to ferroelectric detector array, more specifically, to a ferroelectric detector array utilizing a lead iron-tantalate-niobate alloy and its fabrication.
2. Description of Related Prior Art
Certain crystals exhibit a property to produce a state of electrical polarity with a change in temperature, which is described as the pyroelectric effect. Ferroelectric materials are a type of pyroelectric materials where this polarization, described as a permanent spontaneous electrical polarization, can be reversed by an electric field. The spontaneous polarization is reversible due to the derivation of the crystal from a nonpolarized structure by small displacements of ions. This nonpolarized structure becomes stable if the crystal is heated above a critical temperature, known as the Curie temperature or transition point. The crystal undergoes a phase transition from the polarized or ferroelectric phase, into an unpolarized or paraelectric phase at the Curie temperature. During this transition, large anomalies occur in the dielectric constant such that small changes in temperature of the material result as large changes in the dielectric constant. The temperature dependence of the spontaneous polarization corresponds to a strong pyroelectric effect which can be exploited for different applications.
Ferroelectrics are utilized as electromechanical transducers, light modulators, optical information storage, and in particular, thermal image detectors. Thermal image detectors require detector materials which detect small changes in temperature detectable electronically for which ferroelectric materials are ideally suited. With such a material changes in thermal energy can be detected as the material passes through the transition point, which also results in large changes in the dielectric constant or electronic capacitance. These dramatic changes in values constitute an electronic signal. When this material is made into an array of small detectors, the resulting electronic signals from all the detectors form an electronic image of the thermal scene. The correct choice of ferroelectric material and its fabrication technique is a primary consideration for a ferroelectric detector array utilized in a thermal imaging system.
The transition point of typical ferroelectric materials are extremely low such that a cooler is required to bring the detector material to its transition point (Tc) to be utilized although that point may be below the desired operational range to be useful. The prohibitive size and weight a cooler would add a consideration in the design of a thermal imaging system. By selecting proper amounts of two related ferroelectric materials (compositionally tuning Tc) the solid solution of the two materials may be fabricated as a ferroelectric alloy material which has a specific Curie temperature or transition point. One example of this type of material is barium strontium titanate (BST) which has a Tc of 25 degrees Celsius that is the resultant alloy of barium titanate having a Tc of 120 degrees Celsius with strontium titanate having a Tc of -220 degrees Celsius. The principal difficulty with materials such as BaTiO.sub.3 and BST, discussed above, is that their high dielectric constant reduces the figure-of-merit (FOM) and in turn the ultimate device performance. Other ferroelectric materials such as PbTiO.sub.3, PZT, and PZT do no have the feature of having Tc compositionally tunable, in spite of a sufficient FOM. There is a continual search for new ferroelectric materials utilized in ferroelectric detector arrays which advance the state of the art in sensitivity and resolution of small, lightweight, uncooled thermal imaging systems.
While the prior art has reported using ferroelectric materials as thin film detector array materials, none have established a basic for a specific material and its fabrication that is dedicated to the task of resolving the particular problem at hand.
What is needed in this instance is a lead iron-tantalate-niobate alloy detector array utilizing a ferroelectric with a compositionally selectable Curie point and low dielectric constant.